Re: [Xen-devel] PV drivers on HVM using Xen 4.1.1

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 27 Oct 2011, Alex Bligh wrote:
> > you should use hda here because you want to
> > make sure that an emulated IDE disk is created for you as well.
> 
> On Xen3.3 (and apparently on Xen 4) this happens anyway
> 

Yes, but I wouldn't rely on that: if you specify xvda it means you want
*only* a pv disk. However if you don't have any IDE disks configured,
qemu realizes that you made a mistake in the config and setup one for
you anyway.


> > Also if /tmp/centos-pvd.img is a raw file, you might as well use file:
> > rather than tap:aio.
> 
> There seems to be some doubt (see Ian's message) about whether this
> changes the backend driver that is used. The final deployment application
> is tap:aio with a block device, so that's why we're doing this.
 
If you are using XL, no matter if you specify tap:aio or file:, you are
going to get qemu as disk backend if you are missing blktap.
There is nothing wrong with that, except that qemu in 4.1 doesn't
support linux aio so the performances are not very good. I am not sure
which one is better: blkback on a loop device or qemu without linux aio,
they are both rather slow.

In order to make it fast you can:

- use a dom0 kernel that provides blktap;

- use LVM with blkback;

- use upstream qemu with linux aio as device model and/or block backend.

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